1. Field of the Invention
The present invention relates to output circuits for sequentially receiving and then converting charge collected in the photoelements of an image sensor into an output voltage.
2. Description of the Prior Art
The resolution of image sensors is increased by increasing the number of photoelements of such sensors. As the number of photoelements increases, their size necessarily decreases for a given sensor image area. Charge is collected by these photoelements and transferred to an output circuit which includes a floating diffusion (FD). This output circuit is adapted to convert charge into an output voltage. As the size of sensor elements decreases, the number of signal charge they collect also decreases. Unfortunately, the portion of the output signal caused by noise actually increases. Noise therefore becomes a more significant component of any output signal. Consider that we have a given photoelement signal charge (Q.sub.sig) as determined by its responsivity and signal exposure level. Such charge is independent of the sense node's capacitance (C.sub.FD). Also consider that the source-follower output circuit as described herein has a given input-preferred noise voltage (Vi.sub.SF.sup.2) primarily due to thermal noise of the first-stage drive transistor. Thermal noise is also independent of C.sub.FD for a given transistor width-to-length ratio (W/L) as determined by drive requirements. (Vi.sub.SF.sup.2) can be converted to an input-referred noise in terms of charge by the expression, EQU Qi.sub.SF.sup.2 =C.sub.FD.sup.2 Vi.sub.SF.sup.2.
Another noise term associated with the described output circuit is termed reset or kTC noise, and is given in terms of charge by the expression, EQU Q.sub.kTC.sup.2 =kTC.sub.FD,
where k is Boltzman's constant and T is the temperature. Therefore the signal-to-noise ratio (S/N) is given by, ##EQU1## where the other terms are also independent of C.sub.FD. From this expression, it can be seen that the signal-to-noise ratio is improved by decreasing C.sub.FD.
it is therefore an object of the present invention to provide a highly-sensitive floating-diffusion electrometer design which minimizes input capacitance and thereby reduces noise in the total output signal voltage.